It is known to squeeze a flowable, pasty mass such as sealing mass or dental impression mass out of a tubular bag in that the latter is introduced into a cylindrical space which is acted upon from one end by a plunger and has a discharge opening or ejection nozzle at the other end, at which the bag has been opened. In this case, particular value is placed upon the sealing of that end of the tubular bag which contains the bag opening in relation to that end of the cylindrical space which forms the discharge opening and the ejection nozzle. If, for this purpose, the tubular bag is fixedly connected to the unit forming the cylindrical space (FR-A 1 161 905), the multiple use of the squeezing-out unit for a plurality of tubular bags is made impossible. To avoid this disadvantage, an arrangement for squeezing out from exchangeable tubular bags has become known (EP-A 541 972), in which arrangement the gathered end of the tubular bag, which end forms the bag opening, is fixedly connected to a sealing ring which has a conical sealing surface which cooperates with a corresponding conical sealing surface in the discharge opening of the squeezing-out unit. Under the action of the squeezing-out plunger, the cone of the sealing ring is pressed into the conical discharge opening, provided that it has been adequately centred in advance. If the centring is inadequate, no sealing takes place. If the cone of the sealing ring sits precisely centrally in the discharge opening, it appears to be possible to achieve an adequate sealing action as long as the action of the plunger force persists. However, this sealing action ends when, upon termination of the plunger force, the arrangement "breathes". Since, during squeezing out, the greatest part of the pressure drop does not occur in the bag opening or the discharge opening of the squeezing-out unit, but in the downstream spaces of the nozzle and of the possibly present mixing device, a considerable pressure builds up, during squeezing out, in these spaces downstream of the discharge opening, which pressures leads to corresponding expansion of the associated wallings. When the squeezing-out force of the plunger ends, this expansion then leads to a back pressure and a back flow of the mass in the region of the discharge opening. As a result of this, the cone of the sealing ring may be lifted off from its conical seat in the discharge opening; the sealed gap opens and the mass can penetrate into the space to be sealed off between sealing ring and tubular bag on the one hand and cylindrical space on the other hand. Furthermore, the mutual centering may be lost, so that the cone of the sealing ring is not passed back into the correct seat upon the next application of pressure. Accordingly, contamination of the unit cannot be entirely avoided by the known arrangement. In addition, there is the disadvantage that in the case of the exchange of the tubular bag the discharge opening must be carefully cleaned, because otherwise residues of the mass which have remained there and possibly solidified make a complete sealing off impossible, even during the action of the squeezing-out force; this then leads to further contamination of the unit. Furthermore, the known mode of sealing off requires careful production and assembly of the parts participating in the sealing off; this being costly. In another known arrangement for squeezing out from exchangeable tubular bags (FR-A 2 301 306, FIG. 4), a flat sealing ring is inserted between the flat end of the bag, which is provided with a cross-shaped incision for forming a squeezing-out opening, and the end face of the cylinder space, surrounding the discharge opening. This sealing ring is not connected to thy end of the bag. It has therefore been found that it cannot ensure a tight seal in the region of folds in the bag.